Method for joining two objects and corresponding joining element

ABSTRACT

A first and a second object ( 1  and  2 ) are joined with the aid of a joining element ( 8 ) including at least in the region of its distal and proximal ends ( 8.1  and  8.2 ) a thermoplastic material. Two blind holes ( 5  and  6 ) facing each other are provided in the two objects ( 1  and  2 ) and the joining element ( 8 ) is positioned in the blind holes such that its distal and proximal ends ( 8.1 ) are in contact with the bottom faces of the blind holes and such that there is a gap ( 9 ) between the two objects ( 1  and  2 ). This assembly is then positioned between a support ( 3 ) and a sonotrode ( 4 ). The sonotrode ( 4 ) and the support ( 3 ) are forced towards each other, while the sonotrode ( 4 ) is vibrated, thereby liquefying at least part of the material having thermoplastic properties, there, where the joining element ends ( 8.1  and  8.2 ) are pressed against the bottom faces of the holes ( 5  and  6 ) and allowing the liquefied material to infiltrate into pores of the hole surfaces or unevennesses or openings provided in the hole surfaces.

This application is a continuation of U.S. Ser. No. 11/571,633, filed onJan. 25, 2007 and currently pending.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention concerns a method for joining two objects and acorresponding joining element being applicable to the method, whereinthe joining element consists at least partly of a material havingthermoplastic properties and the method comprises the application ofmechanical vibration, e.g. ultrasonic vibration.

2. Description of Related Art

Publications WO 98/42988 and WO 00/79137 describe joining elements whichconsist at least partly of a material with thermoplastic properties andwhich are, for example, pin-shaped. Such a joining element is anchoredin an object of a porous material by positioning it in a blind holeprovided in the object or by forcing it through the surface of theobject and by then applying mechanical vibration (e.g. ultrasonicvibration) to the joining element while simultaneously pressing itfurther into the object. Due to the mechanical vibration, thethermoplastic material of the joining element is liquefied at thesurface of the joining element where this surface is pressed against theporous material and, in its liquid state is pressed into the pores ofthe porous material where it forms an anchorage on re-solidification. Inthe anchorage area, the porous material is interpenetrated by thethermoplastic material such that it constitutes an intricate positiveconnection between the two materials. Application of the mechanicalvibration is effected by positioning a vibrating element, e.g. thesonotrode of an ultrasonic device, on the joining element (e.g. on theend of a pin-shaped joining element), where it protrudes from the objectin which it is to be anchored.

Publication WO 96/01377 describes a method for joining two objects of aporous material with the aid of a dowel-shaped joining elementconsisting at least partly of a material having thermoplastic propertiesand with the aid of mechanical vibration. In each one of the objects tobe joined, a blind hole is provided and one end of the joining elementis positioned in each one of the blind holes. The two objects are thenpressed towards each other in a direction which is substantiallyparallel to a line connecting the two ends of the joining element and,simultaneously, one of the objects is excited with mechanical vibration.

According to WO 96/01377 the two blind holes and the two ends of thejoining element are similar and the length of the joining elementcorresponds substantially to the total depth of the two blind holes. Thetwo ends of the joining element and the two blind holes provided in thetwo objects are conical and the joining element and blind holes aredimensioned such that when positioning the joining element in the blindholes, the joining element does not reach to the bottom of the blindholes and therefore there remains a gap between the two objects. Thejoining action is carried out by positioning the assembly of the twoobjects and the joining element between a non-vibrating support and anelement which is capable of being excited to mechanically vibrate, forexample, the sonotrode of an ultrasonic device. The excitable elementand the support are forced towards each other by e.g. pressing theexcitable element onto one of the objects, and simultaneously, theexcitable element is vibrated. On pressing and vibrating, thethermoplastic material of the joining element is liquefied where it isin contact with the walls of the blind holes and the joining element isforced deeper into the blind holes, whereby the gap between the parts isreduced. As soon as the gap is closed, no more pressure can be appliedto the joining element and the joining action is therefore finished. Thethermoplastic material which, during vibration, is liquefied infiltratesinto the porous object material and anchors the joining element in thetwo objects.

With the above briefly described joining method according to thepublication WO 96/01377 satisfactory results can be achieved in veryspecific cases only. In order to be applicable in a more general manner,it needs improvement.

BRIEF SUMMARY OF THE INVENTION

It is therefore the object of the invention to provide such improvement.This means, it is the object of the invention to create a joining methodand a joining element for joining two (or possibly more than two)objects, wherein the joining element is used like a dowel, i.e. it ispositioned in a recess provided between the two objects to be joined,e.g. in two opposite blind holes, wherein the joining element consistsat least partly of a material having thermoplastic properties, whereinthe method comprises the step of applying mechanical vibrations andwherein the joining element and the method according to the inventionare to improve the dowel-like joining element and the correspondingjoining method according to WO 96/01377 in such a way that, with theirhelp, it becomes possible to satisfactorily join objects not only invery specific cases but much more generally.

This object is achieved by the joining method and the joining element asdefined in the claims.

The invention is based on the finding that the strength of joints madeaccording to the above mentioned joining method as described inpublication WO 96/01377 is limited and that in most cases it is notpossible to improve this strength by combining the teaching of WO96/01377 with the teaching of WO 98/42988, i.e. by designing the joiningelement to be longer than the total of the depths of the two blind holesand by attempting to get a strong anchorage in the region of the twoends of the joining element. In most cases this is due to the fact thateither the joining element is anchored in the object facing theexcitable element in a satisfactory way while anchorage in the otherobject is not satisfactory, or the joining element end in the objectfacing the excitable element or the material surrounding this joiningelement end suffers from overheating, while anchorage in the otherobject is either satisfactory or not even satisfactory. These findingscan be explained at least partly by the fact that the vibration energyavailable at the proximal end of the joining element (end facing theexcitable element) is greater than at the distal end of the joiningelement (end facing the non-vibrating support) and that therefore lessmaterial will be liquefied at the distal end than at the proximal end.This effect needs to be counterbalanced if it is not to render the jointquality unsatisfactory e.g. by unsatisfactory anchorage at the distaljoining element. The named energy asymmetry is due to several factors,of which only two are named. It is due to the proximal end of thejoining element being positioned nearer to the source of the vibrationthan the distal end (less damping). It is further due to the need of themechanical vibration to reach the distal joining element end by beingtransmitted through the joining element itself, wherein thecharacteristics for vibration transmission are not optimal, particularlywhen there is liquefied material at the proximal end of the joiningelement.

One obvious way for preventing the above named asymmetry consists injoining the two objects with the help of the joining element in twosteps, namely positioning the joining element in the recess of the oneof the objects and anchoring it in this recess according to the teachingof the publication WO-98/42988 and then to position the second elementon the proximal end of the joining element and applying the vibrationalenergy to the second object for anchoring the joining element in thesecond object also. However, such two step method needs more time forthe complete joining action and more complicated devices if it needs tobe carried out fully automatically. Therefore, such two step method isnot the inventive solution to the problem.

The quality, in particular the mechanical strength of an anchorageeffected by mechanical vibration as described above, depends inparticular on how much and how deeply thermoplastic material infiltratesinto pores or other suitable openings in the material in which a joiningelement is to be anchored. This feature not only depends on the methodparameters such as applied pressing force, vibrational energy (dependenton amplitude and frequency) being coupled into the system and timeduring which the vibrational energy is applied, but it also depends to aconsiderable degree on the joining element and recess being provided forthe joining element between the objects, in particular on the design ofthe surfaces of joining element and recess being pressed against eachother, on the resistance which the object material raises against beinginterpenetrated, displaced and/or compressed by the liquefied material,and on the capability of the joining element to be vibrationally coupledto the excited element and to transmit such vibration.

According to the invention the effect of the above named asymmetryregarding vibration energy available at the two joining element endsduring the joining action is counterbalanced by at least one of thefollowing measures:

-   -   The two ends of the joining element are different with regard to        design and/or material, e.g., regarding the size of the two        joining element faces which are pressed against the recess        surfaces, regarding the amount of material having thermoplastic        properties being available for liquefaction and/or regarding the        thermoplastic properties of this material (asymmetric joining        element);    -   The two recess faces against which the joining element ends are        pressed (e.g. bottom faces of the two blind holes) are different        with regard to design and/or material, e.g., regarding the size        of the two recess faces against which the joining element ends        are pressed and/or regarding resistance against        interpenetration, compression and/or displacement by the        liquefied material (asymmetric recess bottom faces);    -   The joining element is designed to allow improved coupling of        mechanical vibration from the object facing the excitable        element during at least part of the joining action (asymmetric        anchoring action).

By applying at least one of the above named measures and therewithadapting the joining element and/or the recess provided for the joiningelement, it becomes possible to achieve satisfactory joints for a muchwider range of applications than is possible by using the teaching ofthe publication WO 96/01377. However, for each different application,experiments are to be carried out to find optimum conditions forachieving a joint as desired.

BRIEF DESCRIPTION OF THE DRAWINGS

The following Figures show the method according to the invention andexemplary embodiments of joining elements to be used in the method.Therein:

FIG. 1 shows the method according to the invention on an exemplaryembodiment of a joining element and objects to be joined, wherein anasymmetric joining element is used in connection with a symmetric recess(two identical blind holes);

FIGS. 2 to 9 show further exemplary embodiments of asymmetric joiningelements used in connection with a pair of identical blind holes(symmetric recess bottom faces) or symmetric joining elements used inconnection with a pair of non-identical blind holes (asymmetric recessbottom faces) or a combination thereof;

FIG. 10 shows a further embodiment of the method according to theinvention in which the joining element is equipped for improved couplingcapability (asymmetric joining action);

FIGS. 11 and 12 show embodiments of the method according to theinvention in which the recess is not formed by a pair of blind holes;

FIG. 13 shows an embodiment of the method according to the invention inwhich the joining element has the form of a tongue and the recess theform of a pair of grooves;

FIG. 14 shows an embodiment of the method according to the inventionusing a plurality of pin-shaped joining elements in connection with, onthe one hand, a plurality of blind holes and, on the other hand, agroove;

FIG. 15 shows in detail an asymmetric joining element which isparticularly suited for joining two wooden boards or chipboard elementsto form a corner;

FIG. 16 shows the joining element according to FIG. 15 and furthercomprising a core;

FIG. 17 shows a further joining element similar to the one of FIG. 15;

FIGS. 18 and 19 show two further joining elements whose effect issimilar to the effect of the joining element according to FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows the method according to the invention using a firstexemplary embodiment of an asymmetric joining element and a pair ofidentical blind holes (identical regarding bottom face, not necessarilyregarding depth) provided as recess between the two objects to bejoined. Joining element and objects to be joined are shown in sectionparallel to the direction of the applied pressing force, i.e. parallelto a line connecting the proximal and the distal joining element ends.The two objects are e.g. wooden boards or chipboard elements or consistof another porous material. The assembly is shown both beforeapplication of pressure and vibration (left hand side) and afterapplication of pressure and vibration (right hand side).

For joining the two objects 1 and 2, they are positioned between asupport 3 and an excitable element 4 (e.g. sonotrode of an ultrasonicdevice), the first object 1 on the side of the support 3 and the secondobject 2 on the side of the excitable element 4. The two blind holes 5and 6 provided, one in each object are facing each other, and thejoining element 8 is positioned in the blind holes 5 and 6, its distalend 8.1 sitting on the bottom face of blind hole 5 in the first object1, its proximal end 8.2 sitting on the bottom face of blind hole 6 inthe second object 2. As the joining element 8 is longer than the totaldepths of the two blind holes 5 and 6, there remains a gap 9 between thetwo objects 1 and 2. As mentioned above, the two blind holes 5 and 6 mayhave the same depth or different depths.

The joining element 8 as shown in FIG. 1 consists of a material havingthermoplastic properties and has a distal end 8.1 which differs from itsproximal end 8.2 by a distal bore 10 rendering the distal face areasmaller than the proximal face area and rendering the amount of materialhaving thermoplastic properties smaller at the distal end than at theproximal one. When the excitable element 4 is vibrated (double arrow)and pressed on the second object 2 (single arrow), the vibration iscoupled to the second object 2 to cause liquefaction of thethermoplastic joining element material in the area of the proximal endface of the joining element. Such liquefaction will lessen thevibrational energy coupled into the joining element 8 and thus reducethe vibrational energy available at the distal end face to effectliquefaction. This effect is compensated by the distal end face beingsmaller than the proximal end face resulting in a larger distal pressingforce per unit area which facilitates liquefaction of the thermoplasticmaterial at the distal end and helps to equalise interpenetration ofliquefied thermoplastic into the porous material of the first object 1as compared to interpenetration into the porous material of the secondobject 2 at the proximal joining element end. This means that at theproximal end 8.2 of the joining element 8 more vibrational energy forliquefaction is available but there is less pressure per unit area forpressing the thermoplastic material into the object material than is thecase at the distal end 8.1 where there is less vibrational energyavailable but more force to press the material into the object. If thediffering effects are balanced together with the method parameters(pressing force, vibration energy and application time), the resultingjoint (right hand part of FIG. 1) comprises distal and proximalanchorage areas 15 and 16 of about the same strength, a result whichcannot be achieved using a symmetric joining element in identical blindholes.

The joining element 8 of FIG. 1 consists completely of a material withthermoplastic properties, e.g. of ABS (acryl-butyl-styrene) which mayinclude strengthening fibers or whiskers. If, during vibrating, thewhole joining element is warmed or, in the last instances of vibrating,is brought into a plastic state, it will then shrink upon cooling andcreate a tension which pulls the two objects against each other.

FIG. 2 shows a further exemplary embodiment of a joining elementcomprising a proximal end face which is larger than the distal end face.The joining element 8 is shown positioned in two identical blind holes(before application of pressure and vibration). Same elements aredesignated with same reference numerals as in FIG. 1. Instead of acentral distal bore 10 as shown in FIG. 1, the embodiment according toFIG. 2 comprises two (or more) coaxial cylindrical portions, wherein thedistal portion has a smaller cross section than the proximal crosssection and there is a step between the two sections.

FIG. 3 shows an embodiment of the method according to the invention(only shown before application of pressure and vibration) in which it isnot the joining element which is asymmetric to achieve a larger pressureat the distal joining element end 8.1, but the recess bottom faces. Theinner recess face against which the distal joining element end 8.1 ispressed is smaller than the inner recess face against which the proximaljoining element end 8.2 is pressed. The joining element 8 has the formof a full or hollow cylinder, and therefore its distal end face is of anequal size as the proximal one, but it sits on a smaller surface area ofthe recess, namely only on a shoulder 20 thereof.

FIG. 4 shows a combination of the embodiments according to FIGS. 2 and3, namely a stepped joining element 8 (proximal end face 8.2 larger thandistal end face 8.1) positioned in two different blind holes (blind hole5 in first object 1 with smaller bottom face than blind hole 6 in secondobject 2). The joining element 8 and the two objects 1 and 2 to bejoined are shown before application of pressure and vibration.

Other embodiments of the inventive method in which the effective distalend face of the joining element 8 is reduced by a correspondinglyreduced inner recess face, against which the distal end face of thejoining element is pressed, may e.g. comprise a blind hole bottom facewith a protruding cone or other protrusion on which a substantially evenjoining element end face sits.

FIG. 5 shows another embodiment of the method according to the invention(only distal end 8.1 of joining element 8 in blind hole 5 of firstobject 1 shown; before application of vibration and pressure) in whichagain the effective distal face area of the joining element 8 is smallerthan the proximal one. Here the joining element 8 comprises a distalinsert 21 of a material which is not liquefiable and preferably notporous or considerably less porous than the object material and whichdistal insert constitutes the distal end face of the joining element.Facing towards the proximal end of the joining element 8, the distalinsert 21 comprises a cross section reduction e.g. in form of a shoulder21.1 and a connecting part 21.2 having a cross section which is smallerthan the distal end face. The connecting part 21.2 reaches into acentral bore 10 of a joining element rest part 8′ being made of thematerial having thermoplastic properties, wherein the connecting part21.2 of the insert is held in bore 10 by e.g. a press fit. Bore 10 ispreferably longer than the connecting part 21.1.

On application of pressure and vibration, the distal insert 21 acts asthe bottom face of the blind hole 5, wherein, on the side of the joiningelement 8, the distal face area is reduced by the central bore 10 and,on the side of the hole, the shoulder 21.1 only counteracts the pressingforce. The material being liquefied at the distal end of the joiningelement infiltrates sideways (arrows) into the material of the firstobject 1. This means that the embodiment according to FIG. 5 is acombination of the embodiment according to FIG. 1 and the embodimentaccording to FIG. 4, wherein the advantage of this embodiment is thefact that there is less empty space into which the liquefied materialcan be pressed without much resistance and without anchorage effect andstill the blind hole can have a cylindrical form which is the easiestone to be realized.

FIG. 6 shows a further embodiment of the method according to theinvention (only shown before application of pressure and vibration) inwhich the joining element 8 is asymmetric regarding the thermoplasticproperties of the material. This joining element is used in connectionwith two identical blind holes (symmetric recess). The asymmetricjoining element 8 comprises a distal part 8.3 and a proximal part 8.4wherein the distal part comprises a first material having thermoplasticproperties and the proximal part comprises a second material havingthermoplastic properties and wherein liquefaction of the first materialcan be effected with less energy than liquefaction of the secondmaterial (first material having a lower liquefaction temperature thansecond material), such that about the same amount of material isliquefied with the lesser vibration energy available at the distal endof the joining element in comparison with the material liquefied by thegreater vibration energy available at the proximal end thereof. Thejoint achieved with this method shows equal qualities of the anchorageat the distal and proximal end of the joining element.

A similar effect as achieved with the joining element according to FIG.6 can be achieved with a joining element having identical distal end andproximal end geometries but in which the thermoplastic material at thedistal end is porous, while the thermoplastic material at the proximalend is non-porous (or more porous against less porous), or in which thethermoplastic material at the distal end has a smaller density than atthe proximal end, or in which the thermoplastic material at the distalend contains more of a non-thermoplastic filling material than at theproximal end (or contains such filling material against no fillingmaterial).

FIG. 7 shows a further embodiment of the method according to theinvention (only shown before application of pressure and vibration), inwhich the two object materials in the region of the recess surfacesconstitute the necessary asymmetry by having different propertiesregarding resistance against interpenetration, compression and/ordisplacement by the liquefied material. The material of the first object1 has a higher porosity and/or a lesser mechanical stability than thematerial of the second object 2. The joining element 8 is symmetric andconsists of a material having thermoplastic properties. Due to thedifference in the object materials, a higher percentage of the lesseramount of liquefied material available at the distal joining element endinfiltrates into the material, while at the proximal end, there is moreliquefied material of which, due to the higher resistance presented bythe object material, a lesser percentage infiltrates into the objectmaterial. This again leads to a symmetric joint showing anchorage ofabout the same strength at the distal and proximal ends of the joiningelement 8. The object material difference does not necessarily includethe whole of the two objects. It is sufficient if the two objectmaterials show the named differences in the area of the recess, wherethe joining element ends 8.1 and 8.2 are pressed against the objectmaterial.

FIG. 8 shows a further embodiment of the method according to theinvention (shown only before application of pressure and vibration)which works on the same principle as the embodiment according to FIG. 7.Here, the material of the first object 1 is substantially non-porous butthe recess in this object is provided with suitable geometries as e.g.undercuts 22 into which the liquefied material infiltrates andconstitutes a positive fit connection when re-solidified. Suchgeometries present less resistance against being filled with liquefiedmaterial or necessitate less liquidity of the material than the pores ofthe material of the second object 2, which effect again counterbalancesthe effects of the lesser vibration energy available at the distaljoining element end. Of course, for certain applications it may benecessary to reverse the asymmetric materials, i.e. to have theless-porous or nonporous material with positive-fit geometries on theproximal side, and the more-porous material on the distal side, possiblywith other asymmetric characteristics to permit a bond with satisfactorystrength on the distal side. However, the same principle of positive-fitbond geometry in a less-porous or nonporous material applies.

FIG. 9 shows a further embodiment of the method according to theinvention (shown only after application of pressure and vibration) whichagain works in the same way as the embodiments shown in FIGS. 7 and 8.The two objects 1 and 2 are chipboard elements being coated with acompact layer 30 of e.g. melamine, wherein the chipboard elements are tobe joined to form a corner, i.e. the blind hole extending parallel tothe melamine layers in the first chipboard element 1 and perpendicularto them in the second one 2. The blind hole 6 in the second chipboardelement 2 has a depth such that it reaches substantially to justunderneath the opposite melamine layer 30, such having a bottom facewhich is not porous and which therefore cannot be interpenetrated nordisplaced or compressed by the liquefied material. As seen from FIG. 9,the liquefied material infiltrates sideways into the chipboard materialat the proximal joining element end 8.2 while at the distal joiningelement end 8.1 it infiltrates into the chipboard material much more inthe direction of the pressing force.

FIG. 10 shows a further embodiment of the method according to theinvention (shown only before application of pressure and vibration) inwhich the asymmetry regarding the vibration energy available at thedistal and proximal end of the joining element 8 is not counterbalancedby a further asymmetry of the joining element and/or the recess, but iscounterbalanced by a design of the joining element 8 which improvestransmission of mechanical vibration from the second object to thedistal joining element end in a second phase of the joining action(asymmetric anchoring action). The joining element 8 comprises a centralpart 8.5 carrying a distal end cap 8.6 and a proximal end cap 8.7, thecentral part consisting of a substantially non-porous material beingsuitable for transmitting mechanical vibration (little damping) and theend caps 8.6 and 8.7 consisting of the material having thermoplasticproperties. The end caps 8.6 and 8.7 are fixed to the central part 8.5preferably by a positive fit connection (not shown). At least theproximal end cap 8.7 is designed with a minimum thickness of its facepart such that it contains just enough material to constitute goodanchorage in the second object 2 and to constitute a good connectionwith the central part 8.5.

On application of pressure and vibration to the assembly, as shown inFIG. 10, the material of the proximal end cap 8.7 is liquefied andinfiltrates into e.g. pores of the second object 2 until the proximalend of the central part 8.5 abuts with the bottom face of blind hole 6.Such abutment and the effect of the pressing force will enhancetransmission of vibration from the excitable element (not shown) via thesecond object 2 to the joining element 8 and in particular to its distalend where it will cause, in a second phase of the joining action,liquefaction of the distal end cap material 8.6.

If the central part 8.5 of the joining element 8 according to FIG. 10 ismade of a material having a sufficient mechanical stiffness, thiscentral part 8.5 also serves for improving stability of the effectedjoint against shearing forces (arrows S).

While the embodiments of the method as shown in FIGS. 1 to 9 rely ongeometrical or material asymmetry of joining element and/or recess, theembodiment according to FIG. 10 relies on asymmetry regarding theanchoring action. The proximal and distal anchorages are not effectedsimultaneously, but at least partly in succession.

All embodiments shown in FIGS. 1 to 10 show two blind holes 5 and 6being provided, one in each object. This is not a condition for themethod according to the invention, which asks in a general sense for arecess to be provided between the two objects in which the joiningelement 8 is positioned such that its proximal and distal ends sitagainst inner recess faces and such that there remains a gap between thetwo objects.

FIGS. 11 and 12 show further embodiments of the method according to theinvention, wherein the recess to be provided between the two objects 1and 2 is realized as one blind hole 6 in the second object 2 and aprotrusion 5′ on the first object 1, wherein hole 6 and protrusion 5′have matching cross sections. Depth of blind hole 6, height ofprotrusion 5′ and length of joining element 8 are matched such that whenthe joining element 8 is positioned in the blind hole 6 and theprotrusion 5′ is positioned in the blind hole also, there remains a gap9 between the two objects 1 and 2, which gap corresponds substantiallywith the amount of thermoplastic material to be displaced into theobject material or into corresponding hollow geometries of blind hole 6or protrusion 5′. Of course it is also possible to arrange the blindhole in the first object and the protrusion on the second object.

According to FIG. 11, which shows only the assembly of objects 1 and 2and joining element 8 before application of pressure and vibration, bothobjects comprise a porous material and the joining element 8 has adistal end face smaller than its proximal one (method as shown also inFIG. 1). FIG. 12 shows on the left hand side object 1 with protrusion5′, symmetric joining element 8 and second object 2 with blind hole 6 tobe assembled essentially as shown in FIG. 11 and on the right hand sidethe joint being effected by applying pressure and vibration to theassembly. The second object 2 comprises a porous material and the firstobject 1 a material with no pores or with substantially fewer pores thanthe material of the second object. Instead, the circumferential surfaceof the protrusion 5′ is equipped with grooves 30, running e.g. in acriss-cross pattern and being open towards the protrusion face such thatthermoplastic material being liquefied at the distal joining elementface infiltrates into the grooves 31 and creates a positive fitconnection between the walls of blind hole 6 and protrusion 5′ asindicated with 32 on the right hand side of FIG. 12. As the joiningelement 8 in the present case is a symmetric one, care is to be takenthat the resistance against liquefied material infiltrating into thegrooves 31 remains smaller than the resistance of the second objectmaterial against interpenetration by the liquefied material. If thiscannot be achieved, an asymmetric joining element is to be used (e.g.containing a central bore at its distal end).

In all the embodiments of the method according to the invention as shownin FIGS. 1 to 12, it is assumed that the joining element has the form ofa pin whose cross section may be e.g. round, oval, square, rectangular,triangular or of any polygonic shape with straight, concave or convexsides. It proves to be advantageous to equip the circumferential surfaceof the joining element with axial ribs which contribute guidance of thejoining element in the recess and which add to the connection strengthat the joining element ends by providing additional anchorage material(greater area).

However, it is no condition of the method according to the inventionthat the joining element must be pin-shaped. All FIGS. 1 to 12, beingsections parallel to the direction of the pressing force, can beunderstood also as cross sections through joining elements having alongitudinal extension perpendicular to the pressing force direction,i.e. having the form of tongues, and being positioned in groves havingthe same cross sections as the blind holes shown in FIGS. 1 to 10 or inone blind hole to be closed by a further tongue having the same crosssections as the blind holes and protrusions according to FIGS. 11 and12.

FIG. 13 shows such a tongue-shaped joining element being positioned intwo opposite grooves (recess between objects to be joined). Theillustrated method is substantially the same as the one illustrated byFIG. 1 (asymmetric joining element having smaller distal end face thanproximal one). As indicated by the plurality of double arrows, it isadvantageous to apply vibration to the whole length of object 2 in orderto achieve homogeneous anchorage in the two objects over the wholelength of the tongue-shaped joining element 8.

FIG. 14 illustrates a further way in which a similar joint is achievedas with the tongue-shaped joining element according to FIG. 13. Thefirst object 1 is equipped with a row of blind holes 5 in which aplurality of substantially pin-shaped joining elements 8 are positioned.The second object 2 is equipped with a groove 33 dimensioned toaccommodate the proximal ends of the joining elements. The secondelement is positioned for joining, is then laterally adjusted to fitexactly on the first object 1 and only then mechanical vibration isapplied to the second object 2 for joining the two objects. Theembodiment as shown in FIG. 14 is considerably less demanding regardingaccuracy than an embodiment in which blind holes are provided in bothobjects for a plurality of joining elements, wherein corresponding blindholes in the two parts need to be aligned accurately in two directions.

FIG. 15 illustrates again the method as principally shown in FIG. 1applied for joining two chipboard elements (objects 1 and 2) to form acorner element. As already shown in FIG. 9, the blind hole 5 in thefirst object 1 extends parallel to and about midway between thechipboard surfaces and blind hole 6 in the second object 2 extendsperpendicular to the chip board surface and reaches beyond the middle ofit. The density of chipboard is usually lower in a center region than inouter regions and its mechanical stability is greater in a directionperpendicular to the surfaces than parallel to them. For this reasonthere is a first asymmetry in the case shown in FIG. 14 regarding theobject material, which for the first object 1 has a smaller mechanicalstability and a higher porosity than for the second object 2. Thisasymmetry is more pronounced the more blind hole 6 reaches beyond themiddle of the chipboard element representing object 2. However, thejoining element 8 is designed to be asymmetric also. For improvedmechanical stability as compared with the substantially compact joiningelements according to the previous Figs., the present joining element 8is designed as a cylindrical tube with an inner cross section to belarger at the distal end 8.1 than at the proximal end 8.2, thusrendering the ring-shaped distal face area smaller than the ring-shapedproximal face area and with more material having thermoplasticproperties at the proximal end 8.2 of the joining element 8 than at thedistal end 8.1. Depending on the object material, it proves advantageousto design the face areas not completely even (perpendicular to a joiningelement axis), but rather with a slope relative to a plane perpendicularto the joining element axis which slope forms a sharpened circular edgeat the proximal and distal faces. This edge is pressed into the objectmaterial of the bottom face of the blind holes by the pressing force andinitiates liquefaction. The effective face area is considered to be theone at the level (dash-dotted lines A and B) to which such initialpressing-in is effected.

The circumferential surface of the joining element according to FIG. 15is equipped with axially extending ribs 35.

The joining element according to FIG. 15 can easily be manufactured byinjection moulding. Its middle portion can easily be designed such thatthe centre of gravity is considerably displaced from a position at halfaxial length such that the joining element can be oriented correctly(proximal/distal) by gravity.

FIG. 16 shows a similar joining element 8 as FIG. 15, which in additioncomprises a core 40 which is made of a material having more mechanicalstrength than the thermoplastic material of the outer joining elementshell. The core 40 is e.g. made of wood and serves in particular forimproving the joint stability against shearing forces. The core 4, ifplaced accordingly near to the proximal end of the joining element mayalso assume the function as discussed for the central joining elementpart 8.5 of the joining element according to FIG. 10.

FIG. 17 shows a further example of a joining element 8 according to theinvention, which is similar to the embodiment as shown in FIGS. 15 and16 but has a non-symmetrical cross section.

FIG. 18 shows a further example of a joining element 8 according to theinvention. The joining element is in function similar to the joiningelements as shown in FIGS. 1, 15, 16 and 17 (asymmetric geometry), butit is not hollow and has an angular (square or rectangular) crosssection.

FIG. 19 shows a further example of a joining element 8 according to theinvention. The joining element is in function similar to the joiningelements as shown in FIGS. 15, 16 and 17, but it is designed to have anadjustable axial length. The joining element 8 comprises a distal part8.8 and a proximal part 8.9, both having the form of a hollow cylinderwherein the distal part 8.8 has a smaller diameter and an outer threadand the proximal part 8.9 has a larger diameter and an inner threadadapted to the outer thread of the distal part 8.8. The length of thejoining element 8 is adjusted by screwing the distal part 8.8correspondingly far into the proximal part 8.9. Instead of the threads,the two joining element parts 8.8 and 8.9 can be equipped with snapelements such that they can be snapped together in a plurality ofdifferent axial positions.

1. A method for joining a first and a second object with the aid of ajoining element comprising at least in the region of a distal end and aproximal end a material having thermoplastic properties, the methodcomprising the steps of: providing a recess between the two objects tobe joined, positioning the joining element into the recess such that itsdistal end is in contact with a recess bottom face in the first object,that its proximal end is in contact with a recess bottom face in thesecond object and that there is a gap between the two objects,positioning the first object against a support and positioning anelement capable of being excited to vibrate mechanically against thesecond object, forcing the excitable element and the support towardseach other in a direction to compress the joining element between itsproximal and distal ends and to close the gap, while exciting theexcitable element to vibrate mechanically, thereby liquefying at leastpart of the material having thermoplastic properties, there, where thejoining element ends are pressed against the recess bottom faces andallowing the liquefied material to infiltrate into pores of the recesssurfaces or unevennesses or openings provided in the recess surfaces,such anchoring the joining element ends in the objects, wherein forachieving suitably similar anchorage qualities at the two joiningelement ends, the distal end of the joining element is different fromthe proximal end and/or the recess bottom face in the first object isdifferent from the recess bottom face in the second object, wherein thedifferences are design differences or material differences.
 2. Themethod according to claim 1, wherein a face area of the proximal joiningelement end is larger than a face area of the distal joining element endand the amount of material having thermoplastic properties is larger atthe proximal joining element end than at the distal joining element end.3. The method according to claim 1, wherein the distal joining elementend comprises a material which is liquefiable at a lower temperaturethan the material at the proximal joining element end.
 4. The methodaccording to claim 1, wherein the distal joining element end comprises amaterial which is more porous, which has less density, or which isfilled to a higher degree with a non-thermoplastic filling material thanthe material of the proximal joining element end.
 5. The methodaccording to claim 1, wherein the joining element comprises a distalinsert and a joining element rest part, wherein the joining element restpart comprises the material with thermoplastic properties and the distalinsert is made of an essentially non-porous material and represents thedistal end of the joining element, wherein the distal insert has adecreasing cross section towards the proximal joining element end and isfastened in a central opening of the joining element rest part.
 6. Themethod according to claim 1, wherein, of the recess bottom faces againstwhich the joining element ends are pressed, the one in the first objectis smaller than the one in the second object.
 7. The method according toclaim 6, wherein the recess bottom face in the first object comprises ashoulder against which the distal joining element end is pressed.
 8. Themethod according to claim 1, wherein the material surrounding the tworecess bottom faces against which the joining element ends are pressedare different regarding resistance against interpenetration, compressionand/or displacement by the liquefied material, wherein the material ofthe first object is more easily penetrated and/or compressed ordisplaced than the material of the second object.
 9. A joining elementsuitable for joining two objects using a method according to claim 1,the joining element being pin-shaped or tongue-shaped and comprising amaterial with thermoplastic properties, wherein the joining elementcomprises a distal end and a proximal end, wherein design and/ormaterial of the distal and proximal ends are different.
 10. The joiningelement according to claim 9, wherein a face of the distal joiningelement end is smaller than a face of the proximal joining element endand/or that the distal joining element end comprises less of thematerial having thermoplastic properties than the proximal joiningelement end.
 11. The joining element according to claim 10, wherein thejoining element comprises a distal part and a proximal part, both partsbeing designed as hollow cylinders, the distal part having a smallerdiameter then the proximal part and that fixing means are provided forfixing the two parts together such that the joining element has anadjustable axial length.
 12. The joining element according to claim 11,wherein the fixing means comprise an inner thread on the proximal partand an outer thread on the distal part.
 13. The joining elementaccording to claim 9, wherein the joining element is pin-shaped andsubstantially hollow and that the joining element comprises a thinnerwall at the distal end than at the proximal end.
 14. The joining elementaccording to claim 9, wherein the joining element comprises a distalinsert of a material having substantially no porosity, the distal inserthaving a cross section getting smaller towards the proximal joiningelement end and being fastened in a central opening of a joining elementrest part comprising the material having thermoplastic properties. 15.The joining element according to claim 9, wherein the joining elementincludes a first material having thermoplastic properties at the distalend and a second material having thermoplastic properties at theproximal end, the first material being liquefiable at a lowertemperature than the second material.
 16. The joining element accordingto claim 9, wherein the joining element includes a first material havingthermoplastic properties at the distal end and a second material havingthermoplastic properties at the proximal end, the first material beingmore porous, less dense or more filled with a non-thermoplastic fillerthan the second material.